1. Field of the Invention
The present invention relates to a solid state image sensor having cells capable of accumulating a charge, each accumulating cell of which accumulates a charge in accordance with the intensity of a corresponding picture element.
2. Description of Related Art
FIG. 1 shows a fundamental structure of a picture element of a conventional MOS type image sensor. A photodiode PD is connected through a MOSFET switch SW to a video line VL which outputs a video signal. The video line is connected to a negative supply E via a resistor R, and the video signal is obtained in the form of voltage variation across the resistor R. The voltage variation is caused by charging current when photodiode PD is charged by negative supply E through switch SW. The charging current is dependent on the amount of charge which is generated within photodiode PD.
Photodiode PD is initially charged by negative supply E at a scan timing and is reverse biased. Photodiode PD then receives light to generate a charge within photodiode PD until its next scan timing. As a result, the charge received during a previous scan is neutralized by the light-generated charge and the reverse voltage Vp, shown in FIG. 2(a), decreases in accordance with the amount of light received during one period of scan timing. For example, when no light is present, the voltage drop .DELTA.q.sub.0 is caused by dark current. When the amount of light received is increased, the voltage drop .DELTA.q.sub.1 or .DELTA.q.sub.2 is caused by light-generated charge. When the amount of light received becomes great, the voltage drop saturates. Thus, the reverse voltage Vp is dependent on the amount of light received, dark current, and the scan period during which charge is accumulated. When the switch is turned ON by scan timing, photodiode PD is again charged by the negative supply E and the reverse voltage of negative supply E. At this time, the charging current varies in accordance with the voltage drop across photodiode PD as shown in FIG. 2(d).
Consequently, the charging current varies in accordance with the amount of light received by photodiode PD during one scan period. However, the video lines have distributive capacitance or stray capacitance, and are therefore equivalent to having the video line capacitance connected in parallel with photodiode PD. Thus, the charging current flowing out of negative supply E during the scanning is lower than when the video line capacitance is not existent.
FIG. 3 shows a partial pattern of one picture element. Photodiode PD is formed in the diffusion layer of an aperture 90, and a capacitance portion is formed including another diffusion portion. Connected to photodiode PD is a MOSFET switch SW. P-type diffusion layers form a source 92 and a drain 93, respectively, and are formed on an N-type substrate 91. A gate is designated by 94. Drain 93 is connected to a video line VL through polysilicon 95. The video line capacitance includes the stray capacitance formed by polysilicon 95, the diffusion capacitance of drain 93, and the stray capacitance formed by the aluminum video line VL.
Conventional MOS image sensors, as described above, have a drawback that the level of the video signal is lowered by the video line capacitance, thus causing lower sensitivity. In addition, since the photodiode of each picture element is charged sequentially with a fixed scan period during which the charging current is detected as a video signal, the point of time of scanning each picture element is different from the others depending on when in one scan cycle the picture element is scanned.
Consequently, the timing for the sample-and-hold of the video signal varies from one picture element to another, causing a problem that the video signal can not be obtained with charges of all picture elements accumulated with respect to the same time reference. If the video signal can not be obtained with charges of all picture elements accumulated with respect to the same time reference, then a spike is produced in the video signal of light sources such as fluorescent lamps whose intensities vary greatly in much shorter periods than the scan period, thus causing problems in measuring average brightness.